The invention relates to a method of controlling the operation of a packet switched CDMA telecommunication network, wherein N network users are connected to the network by a respective terminal, and each terminal communicates by means of a transmitter and a receiver with a receiver and a transmitter of another terminal via a CDMA channel forming the transmission path, and wherein a substantially orthogonal receiver code is assigned to the terminal of each user, which code is used by the other terminals of the network for addressing and encoding packets to the particular terminal.
In a packet switched network or a packet network, data is addressed and provided with control information and then transmitted in packets of specified format, the data transmission line being assigned to the transmission of a single data packet at a time, whereafter the transmission channel is assigned to other transmission functions.
Packet switched telecommunication networks are used widely in computer communications, digital telephone systems and mobile communication networks. As compared with the previous circuit switched network, the packet switched network enables a more efficient utilization of the available frequency band and other telecommunication network resources. The packet switched network is particularly applicable in burst transmission in which the data to be transmitted consists of short data periods and long idle periods during which no data is transmitted between the communicating parties. In such operation several slightly loaded transmission lines are replaced by a single transmission line which is shared by a number of different users, and so the users of the network transmit data via a common transmission line.
A packet network employing code division multiple access (CDMA) provides each user with a code, and these codes, orthogonal with respect to each other, are used to encode data packets to be transmitted. In the CDMA packet network, all users share the same available frequency band. It is important in which way the used codes are associated with each user and in which way they are assigned to the different users. These two functions are usually performed in accordance with a special code spreading protocol. CDMA packet networks employ code spreading protocols of different kinds, such as the common code (C) protocol, the receiver-based (R) code protocol, the transmitter-based (T) code protocol, the common transmitter-based (C-T) code protocol and the receiver-transmitter based (R-T) code protocol. The naming of the code spreading protocol depends on the assignment of the code, that is, on the function with which the code is associated. In the R code protocol, for instance, the network terminal of each user is assigned a code which the other network users use when encoding and transmitting data packets to the particular user, that is, to the receiver in the same network.
However, when using a conventional code spreading protocol, an adequate performance is not achieved, especially as far as the throughput is concerned. The present inventors have observed that none of the above-mentioned conventional code spreading protocols provides a throughput efficiency higher than 0.36. As the traffic load of the network increases, the network is more probable to get into a backlog state in which the throughput is extremely low and long delays occur in the network. Thus, the present-day conventional code spreading protocols do not provide sufficiently good results, which is due to the fact that the packet transmissions are started at random, and so packet collisions are inevitable with increasing traffic load of the network.
To eliminate the above-mentioned problems, it is previously known to use a channel load sensing protocol intended especially for a broadband CDMA packet network employing the R code protocol. However, the codes used in the above-mentioned prior art method and in protocols used in other CDMA networks are not always fully orthogonal with respect to each other, and so the cross-correlation between two codes may also deviate from zero. The probability and level of cross-correlation increase with the number of codes of different users in the channel. In the above-mentioned prior art channel load sensing method, the level of cross-correlation is monitored on the receiver side, thus obtaining a rough estimate of the number of users in the network at any given time. If the level of cross-correlation exceeds a predetermined value, i.e. a CDMA threshold, the transmitter remains waiting for a reduction in the level of cross-correlation. The channel load sensing protocol cannot, however, determine which receiver and/or transmitter is busy at a given time, but it only determines the number of active users. The channel load sensing protocol also operates poorly when the level of cross-correlation is less than the CDMA threshold value, whereas the user, that is, the terminal to which the transmitter wants to transmit a packet, is busy, and so collisions of packets inevitably result after transmission. In order words, the prior art protocol described above is "blind".
Known methods also include the Carrier Sense Multiple Access (CSMA) protocol, i.e. a contention bus, the purpose of which is to prevent random transmission of packets by sensing the bus for the same carrier frequency before a new packet is transmitted. Packet transmission is initiated if the same carrier frequency is not detected in the bus. The CSMA protocol reduces packet collisions but it is applicable to narrow band packet networks only. The CSMA protocol is not either in other respects well suited for use in conjunction with the CDMA network, because the simultaneous transmission of several packets, which is possible in the CDMA network, cannot be detected on the same carrier frequency in accordance with the CSMA protocol.
A further known protocol is the Busy Tone Multiple Access (BTMA), in which each busy receiver in the network at a given time indicates its busy state by transmitting a busy tone signal in a separate busy-tone channel. The BTMA protocol is applicable only to narrow band packet networks.